"心" - is a simplified drawing of following diagram. We have to say that Chinese ancestors must have known clearly the Cardiac structure.
Heart
So "心" means Heart, as time passes, extended abstract meaning were added to "心" as well, like "Feelings" and "Emotions"

And as time passes, not only "心" is to have more meanings, but its writing has been continuously simplified. Seal Script , then Clerical Script and finally at around later stage of Clerical Script , it became almost the same as what we are using now - "心".

As usual let us enjoy a Chinese Calligraphy with "心" inside:

手把青秧插满田，低头便见水中天。心地清净方为道，退步原来是向前
Have you gotten any ideas on how to create a Chinese character to mean "100 million"?